Dual layer electrophotographic elements using a light sensitive pigment dispersed in a polymeric binder have gained widespread use in commercial copiers and printers, especially those using a laser or LED as the digital light for the exposure. One class of pigment widely used is phthalocyanine. The pigments are coated from dispersions typically comprising the pigment, a binder, solvent and surfactants. The coating dispersion requirements are numerous. Besides the functional requirement of light sensitivity, the coated layers need to be uniform and need to adhere to adjacent layers and be stable for thousands of cycles. For dip coating applications and the like, the pigment coating dispersion needs to be free of flocculation at the time of coating.
In general practice, pigment-coating dispersions may be left unused for a period of time between coating operations. With the passage of time, even the most stable coating dispersion will flocculate to some extent. For high quality applications minimum flocculation at the time of coating is problematic. Desirably, it should be possible to redisperse the coating composition by some practical means such as on-line ultrasound (sonication) or high shear energy, both of which are typically used. Thus it is essential that the coating dispersion be stable through these treatments.
The stability of particulate dispersions depends on the ability to disperse the particles in the presence of a stabilizer and to provide access for the stabilizer molecules to adsorb to the pigment surface. If the dispersion process is inadequate, agglomerates will be present. These larger size aggregates are sensitive to high-energy dispersion processes, such as ultrasound. The momentum of the aggregates imparted by the high energy can result in collisions between aggregates to form larger aggregates and ultimately destabilize the dispersion. If the dispersion is finely divided into primary particles or relatively small aggregates, ultrasound will not negatively impact the aggregate size or will actually break up the aggregates into smaller sizes.
There are several factors that influence the stability of the dispersions. These include the dispersing energy (milling time and intensity), the adsorption energy of the stabilizing entity to the particle surface, the molecular weight of the stabilizer (this influences the adsorption kinetics as well as the steric stabilization). If the coating solution contains more than one soluble entity (i.e. two or more polymers), at least one of them should adsorb to the particle surface to provide stability. In this case it may be beneficial to carry out the dispersion step in the presence of the adsorbing polymer alone and subsequently add the non-adsorbing polymer.
The presence of aggregates affects the coating process as well as the quality of the final coating. Particularly in processes like dip coating and ring coating, the amount of fluid deposited is a strong function of its viscosity. The state of the dispersion directly impacts the viscosity, particularly at low shear rates. If the dispersion is flocculated, the aggregates are constantly being destroyed and reformed at the low shear rates. The energy dissipated in breaking up the flocculants manifests as a higher viscosity. (At high shear rates, the time constant of flocculation formation is longer than the rate of fluid deformation, thereby having a lower impact on viscosity at these rates). Thus, the rheological profile of an unstable dispersion is highly shear thinning and tends towards an infinite viscosity as the shear rate approaches zero, i.e., the presence of a yield stress according to the Herschel-Buckley equation. The effect of sonication on these dispersions is to make the aggregates larger and increase the shear thinning behavior. Stable dispersions can also exhibit shear thinning behavior, particularly when the volume fraction of the dispersed phase is greater than 0.2. In the case of stable dispersions, the viscosity tends to level off at low shear rates as opposed to the climb exhibited by unstable dispersions.
Most coating operations, particularly dip coating operations, are carried out at relatively low solids content (less than 5%) in the coating fluid. At this solids concentration, stable unflocculated dispersions are expected to show Newtonian behavior. Newtonian behavior is defined as a dispersion in which the viscosity does not change with the shear rate. When sonication is used to redisperse such solutions, it is desirable that the sonication not change the rheological behavior significantly. Unfortunately it has been observed that in many instances the application of sonification does result in detrimental changes in the rheological behavior of the dispersion.
In U.S. Pat. No. 5,238,764, Molaire, et al described a method of making a dispersion of titanyl fluorophthalocyanine (TiFOPc) consisting of: milling the pigment in the presence of MAKROLON (trademark of General Electric Company, Schenectady, N.Y.), a polycarbonate binder, and a solvent using steel shot for three days, separating the pigment grind from the steel shot; and, adding the isolated mill grind to another preformed solution containing another polycarbonate binder (LEXAN, a trademark of General Electric Company, Schenectady, N.Y.), a polyester binder, and two aggregating dyes, to form a coating composition. The dried layer thickness obtained from the coating composition was about five microns.
Molaire et al, in U.S. Pat. No. 5,614,342 described a process of making a dispersion of a co-crystalline mixture of titanyl phthalocyanine (TiOPc), and TiOFPc consisting of milling the co-crystalline pigment mixture in the presence of a polyester and tetrahydrofuran solvent, using 2 mm steel shot in a Sweco Vibro Energy grinding mill; removing the steel shot; and, adding the resulting pigment dispersion to a solution of the same polyester in the same solvent to provide a coating composition, used to coat a 0.5-micron thick charge generation layer.
In U.S. Pat. No. 5,733,695, Molaire, et al. disclosed the use of certain polyester ionomers as a binder for charge generation layer (CGL) dispersions. The polyester ionomers were demonstrated to impact excellent interlayer adhesion to the coated CGL.
In U.S. Pat. No. 6,057,075, Yuh, et al. describes a method for fabricating a photoreceptor including preparing a first stable coating dispersion including a solvent, a first polymer, and a charge generating material; and diluting the concentration of the charge generating material by adding an amount of a second polymer to the first stable coating dispersion without losing the dispersion stability thereof, thereby resulting in a second stable coating dispersion. Preferred binders include polyvinyl butyral with hydroxyl content greater than about 17%, and molecular weight from about 90,000 to about 250,000. U.S. Pat. No. 6,057,075 discloses the preferred rheological behavior of CGL coating solutions.
The foregoing patents are hereby incorporated in their entirety by reference.
The general equation that describes particulate solutions is the Herschel-Buckley equation: τ=τ0+mγP-1 where τ is the shear stress, γ is the shear rate, m is a constant obtained by fitting and P is the power law index. τ0 is the yield stress that is usually present when the particles are flocculated to form a network structure. In the absence of any yield stress, the equation becomes:τ=mγP-1
According to U.S. Pat. No. 6,057,075, if the solution has no yield stress and has a power law index over 0.9 (1.0 being Newtonian), the solution is stable. Some of the stable solutions disclosed in U.S. Pat. No. 6,057,075 have power law index values as low as 0.92. However, it has been found that solutions that are Newtonian or close to Newtonian (P>0.99), while apparently stable, may be degraded upon being subjected to sonication. For coating solutions where the solids concentration is less than 5%, if the ratio of the low shear viscosity (0.5 s-1) and the high shear viscosity (3000 s-1) is greater than 3.0, it is indicative of a flocculated system and sonication will degrade the solution. In some instances, particularly when the solids concentration is less than 3%, even if the ratio is close to 1, it may not be immune to sonication and results in the rheology becoming non-Newtonian after sonication. Thus, it is desirable to have CGL solutions to have low shear thinning properties upon preparation, but to also retain or improve its shear thinning properties after sonication.